The present invention relates to capacitive sensors, more particularly to methods and devices for identifying or neutralizing, in capacitive sensors, measurement errors involving offset drifts associated with environmental factors such as temperature and humidity.
A capacitive displacement sensor as conventionally known is a noncontact device that uses the electrical property known as “capacitance” to measure position, and/or change of position, of a conductive target. The capacitance that is implemented by a capacitive displacement sensor is that which exists between two conductive surfaces that are sufficiently near each other to establish a capacitance therebetween. The capacitance varies in accordance with variation of the distance between the two conductive surfaces. Therefore, a change in capacitance is indicative of a change in position of a conductive target. For instance, a change in capacitance can translate into a distance measurement.
Diverse applications of capacitive displacement sensors include processing, precision assembly, precision measurement, metrology, etc. A capacitive sensor can be used, for example, in association with a fuze, safety, or arming device. Some conventionally known capacitive sensors are of a MEMS (micro-electromechanical system) variety, for instance machined in silicon and used for various MEMS applications such as measuring acceleration or rotation in an automobile or toy (e.g., a video game controller).
Conventional capacitive sensors are frequently designed to be to “zeroed” to eliminate a constant offset error (e.g., in which a constant value is added to the output voltage) with respect to the original calibration. For instance, these sensors often have the capability to be initialized upon power up through a self-calibration process and polling. However, after the device is zeroed, other kinds of offset error may be introduced into the capacitive sensing due to environmental factors such as temperature and humidity. The device may be subject to offset drift caused by these and other environmental factors and changes thereto over time.
Conventional approaches to compensating for environmental effects involve integration of a capacitive sensor with temperature and/or humidity sensors. Since the temperature and humidity sensors necessitate additional hardware and software, these approaches to environmental compensation may not be suited for some applications, such as safety-critical or volume-critical applications. Moreover, some applications, such as weapon safety systems, are not amenable to powered self-calibrations or polling; in these systems, the device is powered and the sensors report back the status.
It may be the case, for a given application, that an environmentally uncompensated capacitive sensor is adequate insofar as meeting the fidelity requirements of the application. Nevertheless, many applications require higher fidelity, and hence compensation for environmental effects is needed. The importance of compensation for offset drift lies in the fact that significant offset drift can be misinterpreted as a change in the physical quantity to be measured, rather than being correctly interpreted as an electrical bias of an uninfluenced sensor. Accordingly, an improved methodology is sought that captures or negates offset drift that is associated with environmental effects.